1. Field of the Invention
This invention relates to a hydraulic pressure control apparatus for use in an automatic transmission for an automobile.
2. Description of the Prior Art
In the automatic transmission, a lock-up clutch is provided parallel to a fluid torque converter in a passage for transmitting the engine power so that the lock-up clutch is engaged at high vehicle speed to prevent the transmission loss due to the fluid torque converter. However, when the vehicle speed is reduced from high vehicle speed by utilizing an engine brake and the lock-up clutch is under the engaged condition in spite of the high vehicle speed, shocks occurring during deceleration are unfavorably increased and thus the lock-up clutch is desirably under the released condition in a low throttle position in which a throttle valve is almost fully closed in an intake path. Further, in a hydraulic pressure control apparatus, many desirable special controls other than the control of the lock-up clutch are provided for the low throttle position
Also, in a prior hydraulic pressure control apparatus is provided only one throttle valve for producing the throttle pressure related to the intake throttle position and the common throttle pressure is used for the control pressure of a line pressure producing valve and a shift valve. However, since the throttle pressure characteristics for producing a proper line pressure are different from the throttle pressure characteristics for producing a proper speed change point, it is difficult to control properly both the line pressure and speed change point.
Thus, this applicant disclosed in the Japanese Patent Application No. Sho 59-289 (corresponding to U.S. application Ser. No. 689,305, now U.S. Pat. No. 4,691,594) a hydraulic pressure control apparatus in which two throttle valves having respectively individual throttle pressure characteristics are provided to control respectively the line pressure producing valve and the shift valve by separate throttle pressures so that the line pressure and the speed change point are to be both properly provided.